This is my experience as well. I believe they are used to ticketing/pricing ex-US RTW fares at the AA desk, with other origin points being less common. After pricing out my ex-OSL DONE4, the AAgent went on to say that there was something wrong and how the base fare should be 10k+.
After I pointed out that this was originating in Oslo, not the US, and that the fare is legit.. she then corrected herself. I'm guessing that the AA RTW desk isn't aware of how terrible their Oneworld partners are with RTW ticketing.. otherwise, we would all be going through the carrier of the origin region. Times have changed.